This invention relates to ring laser angular rate sensors or ring laser gyroscopes. Particularly, the invention relates to dithered ring laser angular rate sensors, and specifically an apparatus for filtering out dither signals in the output thereof.
In a ring laser angular rate sensor, two electromagnetic waves or beams of monochromatic light traverse an optical closed-loop path in opposite directions. The path encloses an area of a plane. If the sensor is caused to rotate about an input axis which has a component normal to the plane, the frequency of one of the beams will be increased while the frequency of the other of the beams will be reduced. The direction and rate of rotation about the input axis can be measured from the beat frequency, i.e. the frequency difference between the beams. Techniques for generating signals representative of the beat frequency are well known and need not be described herein.
Inherent in ring laser angular rate sensors, as is well known, is the phenomenon known as lock-in. One method of avoiding lock-in is to bias the counter-propagating waves or laser beams with a periodic bias. This is sometimes referred to as dithering the optical beams of light. Dithering is described in U.S. Pat. No. 3,373,650 by Killpatrick, entitled "Laser Angular Rate Sensor", assigned to the same assignee as the present application. As described in the above referred to patent, dithering may be applied to a ring laser angular rate sensor by providing a mechanical biasing means for rotationally oscillating the sensor back and forth. Alternatively, dithering of the optical beams may be induced by a Faraday cell which is capable of optically biasing the laser beams directly.
Dithering schemes like those above, as well as others, cause the usual sensor output to contain a component related to the periodic bias. This bias signal component is sometimes referred to as bias spill-over. In some sensor applications, spill-over of the bias signal into the output signal can be tolerated. However, in navigational control applications bias spill-over can cause unacceptable deleterious effects.